I can telnet to localhost:9200 and 'netstat -l' show it running and listening.
According to http://xenforo.com/community/threads/the-xf_search_index-table-can-be-truncated-if-using-xfes.26182/ then I shouldn't need the xf_search_index table.
After performing a vBulletin 3.8 import, I am now completing a search cache index. Whilst this is occurring, I am watching the number of records within the xf_search_index table increase greatly.
Why is this so?